1. Field of the Invention
Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry (MS) is an apparatus which analyzes the structure of molecules by estimating the mass of a molecule ion and a fragment ion. FT-ICR mass spectrometry has become the ultimate standard for high-resolution broadband mass analysis.
2. Description of the Related Art
As shown in FIG. 1, a trap used in the conventional FT-ICR mass spectrometry is generally constituted of a trap electrodes (10, 13), an independent additional electrode (11) including the center of the trap electrodes (10, 13) (so called “a sidekick electrode”), and an excitation and detection electrode (12). The independent additional electrode (11) has been used to improve the storage efficiency of the ions. Generally, after the step of ion activation, a voltage which is same as that of trap electrode (10, 13) is applied to the independent additional electrode (11) (see FIG. 2).
Resolving power in FT-ICR MS is limited by the duration of the time domain ICR signal. Therefore, there have been several approaches to improve trap design, to better understand ion motion, and to increase ion stability in an ICR ion trap. For example, a Penning trap, confines and stores ions by combination of a spatially uniform static magnetic field and a three-dimensional axial quadrupolar electrostatic field. The quadrupolar field ensures that the ion cyclotron frequency is independent of ion location in the trap.
Ions in such a trap exhibit three periodic motions (cyclotron rotation, magnetron rotation, and trapping axial oscillation). Ion stability derives from these motions. Cyclotron rotation results from the Lorentz force on an ion of mass, m, and charge, q, moving in a static magnetic field, B0, and prevents ions from escaping in directions perpendicular to B0. The ion cyclotron angular frequency, ωc, is given by:
                              ω          c                =                              qB            0                    m                                    [                  Equation          ⁢                                          ⁢          1                ]            
The quadrupolar trapping potential has three effects. First, it introduces a linear sinusoidal trapping axial oscillation along B0, at frequency, ωz, thereby preventing ions from escaping along with the axial B0-direction. Second, the cyclotron frequency is shifted downward from ωc to ω+. Finally, there is a new magnetron rotation perpendicular to B., at frequency, ω−. ωz, ω+, and ω− are given by:
                              ω          z                =                                            2              ⁢              q              ⁢                                                          ⁢                              V                trap                            ⁢              α                                      ma              2                                                          [                  Equation          ⁢                                          ⁢          2                ]                                          ω          +                =                                            ω              c                        2                    +                                                                      (                                                            ω                      c                                        2                                    )                                2                            -                                                ω                                      z                    2                                                  2                                                                        [                  Equation          ⁢                                          ⁢          3                ]                                          ω          -                =                                            ω              c                        2                    -                                                                      (                                                            ω                      c                                        2                                    )                                2                            -                                                ω                                      z                    2                                                  2                                                                        [                  Equation          ⁢                                          ⁢          4                ]            
in which α is a characteristic measure of the trap length, and α depends on the trap geometry. Magnetron motion results from the radial electric field gradient generated by the electrostatic trapping potential.
In a typical closed cylindrical ICR cell, the radial electric field is directed outward toward the excitation and detection electrodes (from the inside to the outside of the trap).
The resulting outward radial force destabilizes ions, because the ion magnetron radius increases as ions lose energy by ion-neutral or ion-ion collisions, ultimately leading to radial ejection and limiting the can affect length of time that ions can be held in the trap.
It is important to note that Eqations. 2 to 4 are derived only for a perfectly quadrupolar electrostatic trapping potential. That assumption is valid only near the center of a trap and in the absence of other ions. Under those conditions, the three natural ion motions are virtually independent and ions can be confined for a long period of time without significant loss.
However, collisions with neutrals, deviation from quadrupole electrostatic trapping potential due to truncated or otherwise imperfect trap electrodes, and Coulombic charge interactions destabilize ions axially and/or radially and result in damping of the time-domain ICR signal. Under either of the described conditions, the three ion motions are no longer independent.